Ball joints have been used as vibration isolation joints in an automobile exhaust line. They absorb the vibration of a car body to prevent an excessive stress from acting on an engine exhaust section and an exhaust-pipe joint section.
Various specific joint structures have been proposed and put in actual use. For example, there are joint structures described in JP-A-10-176777 and JP-UM-T-2598461.
A typical structure will be described with reference to FIG. 6.
An exhaust pipe is composed of a first exhaust pipe 1 and a second exhaust pipe 2. The exhaust pipes 1 and 2 are flexibly joined together with a ball joint mechanism B.
The first exhaust pipe 1 guides exhaust gas from an automobile engine to the second exhaust pipe. The first exhaust pipe 1 is called an exhaust manifold. The second exhaust pipe 2 discharges the exhaust gas into the atmosphere.
More specifically, the joint section between the first exhaust pipe 1 and the second exhaust pipe 2 has a ball section 3 shaped like a ball in external form adjacent to the second exhaust pipe 2, and has a flare section 5 near the first exhaust pipe 1. The flare section 5 has an inner curve 4 corresponding to the ball section 3.
The first exhaust pipe 1 and the second exhaust pipe 2 have a first flange section la and a second flange section 2a, respectively. The first flange section 1a and the second flange section 2a have opposed surfaces, respectively. The flange sections 1a and 2a have openings 1b and 2b at opposed positions so that a bolt 8 (described later) is passed through. The flange sections 1a and 2a are fixed to each other with joint mechanisms C.
The joint mechanisms C are provided at a plurality of positions of the first and second flange sections to join the first and second flange sections together.
One of the joint mechanisms C has a nut 7, the bolt 8, and a coil spring 20. The nut 7 is fixed to the periphery of the opening 2b of the second flange section 2a by welding 9.
The bolt 8 passes through the respective openings 1b and 2b of the flange sections 1a and 2a. One end 8a of the bolt 8 is screwed and secured in the nut 7. The bolt 8 is passed through the center of the winding coil spring 20.
The coil spring 20 is arranged between the other end (i.e., head) 8b of the bolt 8 and the first flange section 1a. The coil spring 20 pushes the first exhaust pipe 1 through the first flange section 1a toward the second exhaust pipe 2.
When a bending force is applied between the first exhaust pipe 1 and the second exhaust pipe 2 by the vibration of the car body, the ball section 3 slides in the inner curve 4 of the flare section 5 to allow the first exhaust pipe 1 and the second exhaust pipe 2 to curve. The coil spring 20 keeps the close contact between the ball section 3 and the inner curve 4 to prevent leakage of exhaust gas.
In such an exhaust-pipe joint structure, the coil spring 20 is formed by winding a circular-section wire rod in coiled form. The height of the coil spring 20 is given by n×d when compressed until the wire rods constituting the coil spring 20 come into contact with each other, where d is the diameter of the wire rod and n is the number of turns of the coil spring.
Accordingly, the length of part of the bolt 8 which is passed through the coil spring inevitably must be longer than n×d. This increases the full length of the bolt 8, thus increasing the weight relatively.
Also, because the coil spring is high, the lateral movement increases when a bending stress is applied in use. Also, there is the problem of increasing the contact between the wire rods of the coil spring to increase the moment.
On the other hand, it is the most important object to improve the fuel efficiency of cars in view of recent environmental issues. To this end, automobile makers have been making various improvements including development of high-efficiency engines, improvement in a combustion method, improvement in fuel quality, development of new driving sources such as fuel cells. On the other hand, it is well known that continuous efforts have been made to improve fuel efficiency by weight reduction of car bodies.
In order to reduce the weight of car bodies, strenuous efforts have been made from the weight reduction of car body structures to the weight reduction of various components in grams.
Also the exhaust pipe system that is the subject of the present invention intends to make the exhaust pipe thinner and to make the bolt 8 of the joint mechanism C smaller in diameter. Although it is said that the weight reduction is recently at its limit, further weight reduction is required for the Earth's environment.
The present invention has been made in consideration of the present situation. Accordingly, it is a first object of the invention to provide a coil spring capable of further weight reduction of an exhaust-pipe joint structure and a new exhaust-pipe joint structure using the coil spring. Furthermore, a second object is to provide a coil spring capable of cost reduction of an exhaust-pipe joint structure and a new exhaust-pipe joint structure using the coil spring.